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Reaver View Drop Down
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    Posted: 14 Jun 2022 at 4:00pm
I just installed a brand new IFD 540 and it's working great, except that I'm getting non-stop terrain alerts while on base in the traffic pattern at multiple airports. Once I turn final it gets happy, but surely there's a way to adjust this or acknowledge the alert to not get them constantly? So far this happens on base to runway 30 at KSQL, and also on both downwind and base at O22. In both cases I was direct-to the airport on the 540 (VFR), so it definitely knew where I was going. I'd like to keep the alerts on overall, but this is rather obnoxious. 

I found a few forum posts about having terrain alert issues at private or unpublished airports, but this is happening for charted, public airports. Any help would be much appreciated!
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AviSteve View Drop Down
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You can't adjust FLTA, but when you acknowledge the alert, the aural part of the alert will cease.
Steve Lindsley
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PA23 View Drop Down
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Although I have the FLTA turned of on the IFD, I've run into the same thing with the eTAWS on my transponder (NGT-9000) and I agree sometimes it can be annoying when you have the terrain or obstacle in sight but the box is squawking "caution terrain!". 

I'm assuming the IFD works the same as my transponder, if I acknowledge the warning the unit will no longer alert me to the current "threat" but will come alive if there is another obstacle that comes into view.

I think the warnings are more likely to occur on a VFR approach/pattern into an airport than flying on an instrument approach, if you get the alert on an instrument approach you are probably in trouble!
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SB Jim View Drop Down
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This happens to me often when flying into airports in the mountains. 

I cancel the warning and it comes back quickly. 

The other day it happened flying base into Fresno Chandler, because of obstacles ahead (either a ground based antenna or downtown buildings). 

It’s a great feature for instrument flight, or even night flight, but in CAVU conditions it can be a distraction.

I’m not sure of the best solution; perhaps one of the Avidyne guys can chime in…..
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Originally posted by SB Jim SB Jim wrote:

This happens to me often when flying into airports in the mountains. 

I cancel the warning and it comes back quickly. 

The other day it happened flying base into Fresno Chandler, because of obstacles ahead (either a ground based antenna or downtown buildings). 

It’s a great feature for instrument flight, or even night flight, but in CAVU conditions it can be a distraction.

I’m not sure of the best solution; perhaps one of the Avidyne guys can chime in…..


I think what is happening, is the terrain avoidance system identifies an obstacle as a specific point and that is what you get the alert on, as you move the terrain avoidance identifies a new trigger point and now alerts on that as a new alert.

I'm using my transponder for TAWS (it is certified? tso'd?) and it does the exact same thing, I was flying low over Senaca lake in upstate NY and it kept squawking caution terrain! caution terrain! even though I kept silencing it.

-PA
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Reaver View Drop Down
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Isn't there an exclusion area around charted runways though? I get it if you're flying through the mountains on an approach, but KSQL for example is at sea level next to the San Francisco Bay, and the mountains are several miles to the southwest. I was ~1-mile right base into runway 30 when this happened, which should be well within the alert exclusion area. Same thing for O22 (standard pattern, right base 17, although that at least has mountains closer to the field). 
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PA23 View Drop Down
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Originally posted by Reaver Reaver wrote:

Isn't there an exclusion area around charted runways though? I get it if you're flying through the mountains on an approach, but KSQL for example is at sea level next to the San Francisco Bay, and the mountains are several miles to the southwest. I was ~1-mile right base into runway 30 when this happened, which should be well within the alert exclusion area. Same thing for O22 (standard pattern, right base 17, although that at least has mountains closer to the field). 


Yes it does ....but I think this might answer your question as to when the exclusion inhibits the alerts

PDA is inhibited when inside an FLTA exclusion area and only runs
between 1NM and 5NM from the nearest runway or ARP and
requires the nav mode to be in Approach mode.
This is on page 3-34 of the current pilot guide, page 3-35 goes on with some more inhibited alerts
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Reaver View Drop Down
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Points: 7 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Reaver Quote  Post ReplyReply Direct Link To This Post Posted: 23 Jun 2022 at 3:09pm
Thanks PA23. Forgive my ignorance, but does being direct-to the airport automatically switch to Approach mode as you get close, or does the IFD actually require an armed and activated IFR approach to switch over? If the latter, it seems strange that flying VFR in a traffic pattern of a charted airport would trigger terrain alerts. 
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PA23 View Drop Down
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as I read it, the IFD must be in approach mode, or meet the requirements on page 3-34 for other terrain clearances requirements.
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Reaver View Drop Down
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What I'm saying is that page 3-34 says: 
The ownship position is inside the rectangular (or circular, in the case of no known runway orientation) FLTA Inhibit zone depicted on the 2D map (generates a “FLTA INHIBITED” CAS Advisory message);

A <1mile base in the pattern is definitely inside the described region. From page 3-33:
There is a rectangular suppression area for all runways in the FMS database. Terrain and obstacles inside this suppression area will not generate any FLTA alerts. Dimensions of the FLTA runway suppression area are 2.0 nm laterally and 4.0 nm off each runway end when runway orientations are known or a circle with a radius of 1 nm centered on the Airfield Reference Point (ARP) when the on-board databases do not know the runway orientations. 

But I'm still getting the alerts. 
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Best decision I made was to install a terrain alert inhibit switch, it's in the installation manual.  I was just getting alerts for runway 10 into IYK.  First pattern  I was tight and never climbed to pattern altitude, didn't get an alert.  Second pattern was closer to normal and it was going off on downwind turning final.  I was just VFR doing a few patterns after changing the oil so I didn't have a flight plan loaded.  As soon as that thing started alerting, I just flipped the switch and went on my way.  I also get the alerts any  time I fly west though the pass regardless of having a flight plan loaded.  I don't mind the fact that the alerts would be going off, obviously I'm surrounded by mountains.  I just like to be able to disable the alerts when I know what I'm navigating around.

I was advised to not install the switch for all the safety reasons, but I just put enabling the alerts with the switch in my pre-takeoff and post-landing checklists.
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Originally posted by Reaver Reaver wrote:

What I'm saying is that page 3-34 says: 
The ownship position is inside the rectangular (or circular, in the case of no known runway orientation) FLTA Inhibit zone depicted on the 2D map (generates a “FLTA INHIBITED” CAS Advisory message);

A <1mile base in the pattern is definitely inside the described region. From page 3-33:
There is a rectangular suppression area for all runways in the FMS database. Terrain and obstacles inside this suppression area will not generate any FLTA alerts. Dimensions of the FLTA runway suppression area are 2.0 nm laterally and 4.0 nm off each runway end when runway orientations are known or a circle with a radius of 1 nm centered on the Airfield Reference Point (ARP) when the on-board databases do not know the runway orientations. 

But I'm still getting the alerts. 
The altitude threshold for enroute alerting is 750 feet.  If you were at the published pattern altitude of 800 feet and the system was in enroute mode, then you are pretty close to that threshold and slight descents would cause alerts to be generated.

The 10.2 Pilot's Guide is in error and overstated the size of the exclusion area.  Complicating matters, the performance standards (TSO-c151) have changed since 10.2 software.  The 10.3 software has updated alerting thresholds and those are correctly reflected in the 10.3 Pilot's Guide.  The updated exclusion area extends 0.75nm on each side of the runway and 1.0nm past the ends.  If no runway information is available, it's a 1.0nm area around the airport reference point.

With 10.3, the nuisance alerts should be reduced, but still it depends on the pattern being flown and the surrounding terrain.  In the end, the alerts are there to warn you of a potential terrain conflict, so you don't want to inhibit them so far that you aren't alerted to real danger.  The difficult part is finding the right balance.  As brou0040 said elsewhere in the thread, a terrain alert inhibit switch can be a useful tool when the system just isn't getting it and you as the pilot can say "I've got this".

If you would like to send me your logs, we can take a look at your flight path and at least tell you why you were getting the alerts.  Send me a PM if you want to do this and we'll coordinate.
Steve Lindsley
Avidyne Engineering
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I installed a mid continent terrain module (MD21) that includes an inhibit switch.  It works well when going into unmarked airports like Alton Bay ice runway.  Other than those types of airports, I don’t have it going off at all.  You can find them on eBay for $100.
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Originally posted by brou0040 brou0040 wrote:

...to install a terrain alert inhibit switch, it's in the installation manual.


It would be pretty easy to add a self latching relay (driven by a momentary contact switch), that would reset to OFF when the aircraft power was turned off. That way you would never start up with the suppression engaged. Doesn't help for the rest of the flight, but ...

Even better would be an electronic timer driven circuit, that would reset after a minute or two. Perhaps that was what GRing was suggesting.

* Orest


Edited by oskrypuch - 01 Jul 2022 at 12:37pm
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